1. Field of the Invention
The present invention relates to a process and an apparatus for electrolytic deposition of uniform metal layers, preferably of copper, having given physical-mechanical properties.
2. Description of the Related Art
The electrolytic metallization, for example with copper, of workpieces which are electrically conductive at least on their surface, has been known for a long time. The workpieces which are to be coated are connected as cathode and, together with anodes, brought into contact with the electrolytic plating solution. For the deposition, a flow of electric current is produced between anode and cathode.
Generally, anodes made of the same material as the plating solution are used. The amount of metal deposited from the solution is returned to the plating solution by dissolving at the anodes. In the case of copper, the amount deposited and the amount which is anodically dissolved are approximately the same for a given charge flow. This process is easy to carry out because when copper is used, only sporadic measurement and control of the metal-ion concentration of the plating solution is necessary.
However, disadvantages have been encountered when carrying out the process with these soluble anodes. If the thickness of the metal layers deposited is to be very uniform on the surface of the workpiece, then the soluble anodes are only conditionally suitable for this purpose, because soluble anodes change shape with time due to the dissolving, so that the distribution of the lines of force in the electrolytic bath also changes. The use of smaller, for example spherical pieces of metal as anodes in insoluble metal baskets is also only conditionally suitable for solving the problem, because the metal parts frequently wedge against one another and as a result of the dissolving process, gaps are frequently formed upon the sliding down in the pile of metal parts.
Therefore, attempts have frequently been made to use insoluble, and therefore dimensionally stable anodes, rather than soluble metal anodes, for example, titanium or high-grade steel. Using these anode materials, gases, such as oxygen or chlorine, are formed upon the electrolytic deposition since the anodic dissolving of metal no longer takes place. The gases produced attack the anode materials and gradually dissolve them.
German Patent DD 215 589 D5, describes a process for the electrolytic deposition of metal which uses insoluble metal anodes and to which reversibly electrochemically convertible substances are added to the plating solution. These substances are being transported by intense positive convection with the plating solution to the anodes of a plating apparatus. They are converted electrochemically by the electrolysis current, and then guided by intense positive convection away from the anodes into a regeneration space, returned electrochemically to their initial condition in the regeneration space on regeneration metal present in it with simultaneous electroless dissolving without external current of the regeneration metal, and fed in this initial condition again to the separation apparatus by intensive positive convection. In this process, the above-discussed disadvantages associated with the use of insoluble anodes, are avoided. Instead of the corrosive gases, the substances added to the plating solution are oxidized at the anode, so that the anodes are not attacked.
The dissolving of the metal in the regeneration space is in this case independent of the process of the deposition of metal on the material being treated. Therefore, the concentration of the metal ions which are to be deposited is controlled by the effective metal surface in the regeneration space and by the velocity of flow in the circuit. In the case of a deficiency of metal ions, the effective metal surface and/or the velocity of flow from the deposition space to the regeneration space is increased or, in case of an excess of metal ions, correspondingly reduced. This process therefore presupposes that a high concentration of the reversibly electrochemically convertible substance is present in the plating solution. This results in oxidized compounds of the addition substances (redox system) being again reduced at the cathode, so that the current efficiency is decreased.
German Unexamined Application for Patent DE 31 10 320 A1 describes a process for cation reduction by anode-supported electrolysis of cations in the cathode space of a cell, the anode space containing ferrous ions as reducing agent and anodes that are moved relative to the anolyte which surrounds the anodes.
German Unexamined Application for Patent DE 31 00 635 A1 describes a process and an apparatus for supplementing an electroplating solution with a metal to be precipitated in an electroplating apparatus, wherein the metal which is to be galvanically precipitated is provided in an electroplating solution. The electroplating solution is contained in an electroplating container and a supply of the metal to be precipitated is provided within an enclosed space. Gases produced in the electroplating container upon the advance of the electroplating process are guided, together with the electroplating solution, into the enclosed space and applied there to the supply of metal in order to dissolve the latter. Then, the dissolved supply of metal is again added to the electroplating solution in the electroplating container. The apparatus required for carrying out of the process is, however, very expensive, because it must be gas-tight.
However, the discussed processes have the disadvantage that the plating solutions to be regenerated contain no additives, which, are generally required in order to control the physical-mechanical properties of the deposited metal layers. Such substances are predominantly organic substances.
It is only by these additives that the required physical-mechanical properties of the layers such as, for example sufficient brightness, high elongation upon rupture, and resistance of the layer to cracks upon soldering shock tests are obtained. Without the addition of these additives, the layers are dark, dull, and rough.
DD 261 613 A1 describes a process for the electrolytic deposition of copper from acid electrolytes with dimensionally stable anode using certain additives for the production of layers of copper having specific physical-mechanical properties. The plating electrolyte also contains the aforementioned electrochemically reversibly convertible additives.
Although the quality of the metal layers precipitated from such plating solutions are initially satisfactory, in particular with regard to the physical-mechanical properties, it has been found that after a longer period of deposition, layers of poorer quality are obtained. This is the case even if the substances in the plating solution, whose concentration was decreased by consumption upon the deposition are supplemented. Copper coatings which are only poorly ductile are obtained from used plating solutions. This causes tearing of layers on printed circuits in the region of the drill holes when they are subjected to a soldering shock test. Further, the surface of the metal layer also changes in that it becomes dull and rough.